Assistive Technology: Hearing Aids
What is a Hearing Aid? Parts of a Hearing Aid
Air Conduction Aids Bone Conduction Aids
A hearing aid is an electronic, battery-operated device that amplifies and changes sound to allow for improved communication. Hearing aids receive sound through a microphone, which then converts the sound waves to electrical signals. The amplifier increases the loudness of the signals and then sends the sound to the ear through a speaker.
Here are the parts of a hearing aid.
A: Function Switch: Usually M-T-O (Microphone, Telecoil, Off). Located at the bottom of the aid.
B: Battery: Supplies power to the amplifier. Also located at the bottom of the aid in a swivel-out compartment.
C: Microphone: Converts acoustic energy to electrical energy. Usually at the top of the aid.
D: T-Coil: Converts magnetic energy to electrical energy. Inside the aid, toward the center.
E: Amplifier or Circuit: Increases the strength of the electrical signal. Also filters and modifies the response of the hearing aid to match the hearing loss of the user. Inside the aid.
F: Volume (Gain) control: Allows the user to increase or decrease the over-all level of the sound after the amplifier. Many newer hearing aids have automatic gain control. This is a dial in the middle back of the aid.
G: Receiver: Converts electrical energy into acoustic energy. This is connection point of the hook, on the top of the aid.
H: Sound hook (tonal tube): Delivers sound from the receiver to the earmold. It may be filtered to further adjust the signal to match the hearing loss. This is the plastic device reaching out of th top of the aid.
Here are some more resources concerning hearing aids:
In normal hearing, sound passes along our ear canal to the eardrum making it vibrate. These vibrations are passed to three small bones in the middle ear, the ossicles, a process called air conduction. These in turn pass the vibrations across the oval window to the cochlea and the fluid it contains. Movement in this fluid bends the tiny hair cells along the length of the cochlea, generating signals in the auditory nerve. The nerve signals pass to the brain, which interprets them as sound. A conventional hearing aid makes sounds louder and delivers them to the ear canal. The earmould or aid generally fits the ear canal very closely, so that only sound from the aid enters the ear. These amplified sounds are then heard through air conduction, in the normal way.
In general, there are four main types of air conduction hearing aids: behind the ear (BTE), in the ear (ITE), in the canal (ITC), and completely in the canal (CIC).
A person will be fit with a specific type of hearing aid depending upon his/her hearin loss and preferences.
There is also the above air conduction hearing aids, there is also the bone conduction hearing aid with those who have conductive losses. Today, most conductive losses are corrected through surgery.
- Introduction - Traditional BC Aids - BAHAs - BAHA Implantation - BAHA Extras - Who Benefits from BAHA -
Conventional hearing aids fit in the ear canal and amplify sounds, which the hearing aid user then hears in the normal way. However, these hearing aids are not suitable for everyone with a hearing loss. Some people cannot wear such aids or earmoulds in the ear canal because they have continual infections or eczema, or because part or all of their ear or ear canal is missing. Bone conduction hearing aids offer a solution for them. Additionally, some people who have a conductive hearing loss – an obstruction in their outer or middle ear – may hear better if sounds pass directly through their skull to the cochlea by bone conduction, avoiding the outer and middle ear.
You hear sounds by bone conduction when sound vibrations are transmitted directly from your skull and jaw bones to the cochlea, missing out your outer and middle ears. People always hear some sound – especially their own voices – in this way. If you ever wear earplugs, you will notice that your own voice still sounds fairly loud, because you hear it through bone conduction. Hearing through bone conduction is not as efficient as through air conduction but it is a possibility for people who cannot hear through air conduction.
--Types of Bone Conduction Hearing Aids
There are two main types of bone-conduction hearing aid: the traditional type and the implanted Bone-Anchored Hearing Aid (BAHA).
--Traditional bone-conduction hearing aids
Traditional bone-conduction hearing aids transmit sound to your ear through a vibrating pad placed on your head just behind the ear. Usually you wear a headband to keep the pad in place. Sometimes, the bone conduction aid can be fitted inside a pair of spectacles. The vibrating pad must be held tightly to your head, so these aids are generally uncomfortable to wear. People get headaches and skin soreness because of the pressure from the vibrating pad. An additional problem is that the sound people hear with these devices is often distorted. They also tend to use up batteries quickly, and may be difficult to wear as they can slip. The one pictured to the right is a more modern, aesthetically-pleasing version of the headband bone conduction aid.
An alternative to traditional bone-conduction aids is an implanted Bone-Anchored Hearing Aid (BAHA). A BAHA consists of a small titanium fixture (implant) which is inserted into your skull in the mastoid bone, just behind your ear. The fixture is made of a special material so that the bone bonds with it in two to three months and it becomes securely attached. A small screw (called a ‘percutaneous abutment’) is connected to this fixture through your skin. The picture to the left shows the percutaneous abutment.
The
external part of the BAHA contains a microphone,
battery and processor, and is attached
to the screw. If you have a severehearing
loss, you will use the device with a body-worn
amplifier, a small box that you wear on a belt or in a pocket.
It is connected by leads to the external unit at your head. Sound vibrations
are transmitted directly through the screw to your bone and from there
to the cochlea. The sound is transmitted more efficiently through a
BAHA, and research has shown that peopleusing one hear more clearly
than with a traditional bone-conduction aid. In addition to the improved
sound quality, BAHAs are less visible, use fewer batteries, and are
securely attached. People using them report fewer ear-canal infections
or soreness and very little feedback (whistling or squeaking) from the
BAHA compared with a conventional air-conduction hearing aid. However,
the BAHA may not be able to provide as much power or such a high sound
quality as an ordinary air-conduction hearing aid. An air-conduction
aid is therefore always the better option if possible. People who have
used an air-conduction hearing aid previously may find that the sound
quality and volume are quite different with the BAHA, and this may take
some time to get used to. The picture to the right shows the battery
and processor once attached to the screw.
The
operation to implant a BAHA is normally carried out in two stages and
you may have either a local or a general anaesthetic. Each stage is
a minor operation that takes about an hour at most. First, a small
titanium fixture about 3-4mm long is placed in the bone directly
behind your ear. The consultant will usually pick the side where you
have better hearing, though other factors may affect the decision. For
example, if your hearing is similar in both ears and you drive a lot,
it may be better to choose your left (passenger-side) ear. Alternatively,
it may be better to choose the ear you usually use for the telephone.
The second stage takes place three or four months after the first operation,
when the fixture should have bonded with your
bone. The titanium screw is attached to
the fixture through your skin. For a few
weeks, the part of your head that has been operated on will have to
be kept clean to reduce the risk of infection. Sometimes the whole of
the operation can be done at once. About a month after the second operation
– or longer if the whole of the operation was done at once –
you will get the sound processor. You’ll
be shown how to attach and remove the processor and use all the controls.
The clinic will also explain how to clean the area around the screw.
This daily cleaning is very important and reduces the risks of infection.
Accessories
and features may be available with some BAHAs. These include a directional
microphone, an extra microphone for your other ear, direct audio input
and a telecoil with a ‘T’ switch so that you can use induction
loops. Processors are available in different colours.
--Who might benefit from a BAHA?
Only those people most likely to benefit receive a BAHA. You must have one of the following:
Bone Conduction information was provided by RNID Information Services: http://www.rnid.org.uk/pdfs/factsheet_pdfs/bone_conduction_hearing_aids.pdf.